Avoid SEGV when destroying QCoreApplication.

Remove shared proxies from the static list when they die, so that next
QtSignalForwarder::connect() call does not try to reuse destroyed
object. Instead, it will just create a new one.

This can happen if you create/destroy multiple QCoreApplication objects,
e.g. in tests.
Merge pull request #6 from mrzechonek/master

Avoid SEGV when destroying QCoreApplication.